Recently, the Federal Communications Commission (FCC) has imposed limits on conducted and radiated emissions from unintentional radiators to update the references to Publication 22 of the International Electrotechnical Commission (IEC), International Special Committee on Radio Interference (CISPR). It is difficult for electronic devices, such as devices that include fixed-frequency, switching power supplies to meet CISPR 22, as it requires diligence in component selection and printed circuit board layout.
The operation of a conventional fixed-frequency switching regulator will now be described with FIG. 1. The switching regulator 100 utilizes a LinkSwitch integrated circuit from Power Integrations, Inc., and includes an internal MOSFET switching element. During power up, as VIN is first applied, the CONTROL pin (C) capacitor C1 is charged through a switched high voltage current source connected internally between the DRAIN and CONTROL pins (D and C, respectively). When the CONTROL pin C voltage reaches approximately 5.6 V relative to the SOURCE pin S, the high voltage current source is turned off, the internal control circuitry is activated and the high voltage internal MOSFET begins to switch. At this point, the charge stored on C1 is used to supply the internal consumption of the chip.
As the output voltage, and therefore the reflected voltage across the primary transformer winding ramps up, the feedback CONTROL current IC increases. When IC exceeds IDCS, typically 2 mA, the maximum duty cycle is reduced. R1 is initially selected to conduct a value of IC approximately equal to IDCT when VOUT is at the desired value at the minimum power supply input voltage. When the duty cycle drops below approximately 4%, the frequency is reduced, which reduces energy consumption under light load conditions.
When a fault condition, such as an output short circuit or open loop, prevents flow of an external current into the CONTROL pin (C), the capacitor C1 discharges towards 4.7 V. At 4.7 V, an autorestart is activated, which turns the MOSFET off and puts the control circuitry in a low current standby mode. In auto-restart, the LinkSwitch device periodically restarts the power supply so that normal power supply operation can be restored when the fault is removed.
An optical isolator feedback is used to improve output voltage regulation. To implement the optical isolator regulation, resistor R3, capacitor C3 and optical isolator (transistor) U1 are included on the primary side, where resistor R3 forms a potential divider with resistor R1 to limit the transistor U1 collector emitter voltage. On the secondary side, voltage sense circuit components resistor R5, Zener diode VR1 and LED D2 provide a voltage feedback signal. In the exemplary circuit shown, the Zener diode VR1 reference is used to provide output voltage tolerancing and cable drop compensation, if required. Resistor R4 provides biasing for the Zener diode VR1. The regulated output voltage is equal to the sum of the Zener diode (VR1) voltage plus the forward voltage drop of the LED D2. Resistor R5 is an optional low value resistor to limit LED D2 peak current due to output ripple.
The feedback configuration is a resistive divider made up of resistor R1 and resistor R3 with diode D1, resistor R2, capacitor C1 and capacitor C2 rectifying, filtering and smoothing the primary winding voltage signal. The optical isolator therefore adjusts the resistor divider ratio to control the DC voltage across resistor R1 and therefore, the feedback current received by the LinkSwitch CONTROL pin (C). When the power supply operates in the constant current (CC) region, for example when charging a battery, the output voltage is below the voltage feedback threshold defined by LED D2 and Zener diode VR1 and the optical isolator is fully off.
When the output reaches the voltage feedback threshold set by LED D2 and Zener diode VR1, the optical isolator turns on. Any further increase in the power supply output voltage results in the transistor U1 current increasing, which increases the percentage of the reflected voltage appearing across resistor R1. The resulting increase in the LinkSwitch CONTROL current reduces the duty cycle according and therefore, maintains the output voltage regulation.
Typically, resistors R1 and R3 are equal in value. However, increasing resistor R3 (while reducing R1 to keep R1+R3 constant) increases loop gain in the constant voltage (CV) region, improving load regulation. The extent to which resistor R3 can be increased is limited by optical isolator transistor voltage and dissipation ratings and should be fully tested before finalizing a design.
With a constant load, the tightly controlled feedback loop of the switching regulator 100 keeps the output voltage VOUT of the switcher very stable. This closed loop system maintains a constant switching waveform that results in repetitive, Fourier Series frequency components as shown in FIG. 2.
Many older electronic devices, such as electronic meters, that include wide range power supply modules, use a switching regulator such as that shown in FIG. 1. Typically, these switching supplies were required to meet FCC part 15 emissions requirements for class B computing devices. Achieving a significant margin below the CISPR 22 requirement is very difficult using a fixed-frequency converter because of the sharp, narrow bandwidth harmonics of the switcher. In addition, many electronic devices must minimize hardware cost while maximizing functionality to be competitive. One of the major hardware elements within, e.g., an electronic meter, that has these constraints is the power supply. Typically, an electronic meter supply must work over an extended input AC voltage range, develop sufficient output power to supply a variety of communication options and meet the latest levels of conducted EMI per CISPR 22 rules. These requirements are not easily accomplished while minimizing cost.
Thus, there is significant probability that option boards, component changes and different meter configurations will add frequency elements or conduction paths that will not meet CISPR 22. As such, there is a need for a switching regulator that is both cost effective and that has a reduced emissions to more easily enable electronic devices to meet CISPR 22 requirements. The present invention provides such a solution.